Typically, an exterior insulation layer for use with walls of a building includes a thermal insulating material covered by a rain intrusion protection such as siding or stucco. Thermal insulation material can be impermeable or permeable to water vapor. Stucco can be a three-coat stucco (a three-coat, traditional, metal-lath-reinforced, cladding system) or one or two coat stucco (reinforced, applied in two layers) or a modem synthetic stucco (a thin lamina reinforced with fiberglass or polymeric mesh) used in Exterior Insulation and Finish Systems. Those plasters (stuccos or laminas) provide a barrier to rain entry. While the old masonry buildings used renderings with slack lime as the binder and provided a substantial capillary action, the renderings based on cement generally use polymeric admixtures that further reduce capillarity of the material. Portland cement plaster replaced the traditional renderings.
With the exception of brick veneer, all conventional rain controlling elements in building enclosures are focused on the reduction or elimination of water entry into building materials or components. For example, hydrophobic coatings or other film forming compositions may be applied on the exterior surface of Portland cement plaster to provide low water transmission, while retaining good flame retardancy and low smoke generation of the plaster. Similarly, a coating of polypropylene resin can be applied to the surface of a fibrous sheet to make the sheet impermeable to water and vapor. Subsequent treatment provides vapor permeability to the sheet while maintaining liquid water impermeability. The resultant product is particularly suited for use as a roofing-tile underlayment or as an air-infiltration barrier. Alternatively, water barriers may be coated with other elastomers, including dispersed layer fillers in liquid carriers, or may include a sheet of paper impregnated with asphalt or urethane compounds.
Yet three-coat Portland cement plaster is prone to cracking and subsequent water penetration. On the other hand, some synthetic stuccoes (a thin lamina reinforced with fiberglass mesh) are elastic and less prone to cracking, yet may not provide sufficient fire protection and drying ability to the wall as well as may lack the water storage capacity of the traditional three-coat lime-cement plaster.
Moisture management on the interior side of the exterior walls has been concentrated on water vapor retarder technology. For instance, one vapor barrier type includes polyamide (nylon) fibers that are modified with polyvinyl alcohol. Since these fibers are susceptible to moisture, the water vapor permeance of the membrane changes with relative humidity. Another barrier type comprises a sheet of a unitary, non-woven material that is spun-bonded from synthetic filamentary fibers. The sheet is then textured with protrusions to define channels oriented in multiple directions that provide a means by which a liquid on the first side of the sheet can drain. Various gypsum-based panels used as interior finish function well as an air barrier but do not contribute to water management. On the contrary, those including paper (cardboard) are sensitive to water and must be protected from prolonged exposure to water.